Attitude sensor

ABSTRACT

The present invention provides an attitude sensing device and method for determining an attitude of a reference axis of a package containing a fibre optic sensor sensor. The attitude sensing device comprises an electromechanical attitude sensor for generating an electrical signal indicative of the attitude sensor, and converter logic for converting the electrical signal into a stimulus signal. A local power source is preferably provided for the electromechanical attitude sensor in the converter logic. The stimulus signal is such that the fibre optical sensor is responsive to the stimulus signal to cause a variation in at least one predetermined property of an optical signal transmitted through the fibre optic sensor, the attitude of the reference axis being determinable from the variation of the predetermined property. By this approach, it is possible to use the existing fibre optic sensor within the package, along with the corresponding existing telemetry and multiplexing system, to recover the information from the electromechanical attitude sensor.

FIELD OF THE INVENTION

The present invention relates to an attitude sensing device and anattitude sensing method, and in particular to techniques for determiningan attitude in three dimensional space of a reference axis of a packagecontaining a fibre optic sensor.

DESCRIPTION OF THE PRIOR ART

Fibre optic sensors are becoming a well-established technology for arange of applications, for example geophysical applications. Fibre opticsensors can take a variety of forms. For example fibre optic sensors maybe arranged to act as static pressure sensors or static temperaturesensors. Additionally, fibre optic sensors have also been developed formeasuring dynamic quantities such as acoustic and seismic signals,example of such dynamic fibre optic sensors being fibre optichydrophones and fibre optic geophones. A hydrophone is a device for themeasurement of dynamic pressure in a fluid, whilst a geophone is adevice for the measurement of vibration (in practice, this can either bean accelerometer or a displacement sensor).

An example of an application in which dynamic fibre optic sensors arenow seeing use is in the oil/gas industry. In particular, it has beenrecognised by the industry that fibre optic sensors have a major role toplay in the rapidly growing area of monitoring of oil or gas reservoirs,such an activity often being referred to as reservoir characterisation.

In such applications, arrays of sensors are typically used, the arrayconsisting of a series of sensor packages. The actual arrangement ofsensors placed within each package is obviously a matter of designchoice, but typically each package will include up to three orthogonallymounted geophones (directional vibration sensors) and one hydrophone(omnidirectional pressure sensor). These packages are often known as 4-C(4-component) packages, and one array may contain more than a thousandsuch packages.

In an example deployment, such an array of hydrophones and/or geophonesmay be spread out along a seabed for monitoring of oil or gas reservoirswithin the seabed. To carry out reservoir characterisation, a separateacoustic source is used to transmit seismic signals into the oil fieldstructure, and the array of sensors is used to record the signalsreflected from the various geological layers within the structure.

To interpret the signals generated by the sensors in the array as aresult of such a process, it is important to know the orientation ofeach package, and hence the orientation of the sensors within eachpackage, and in typical deployment conditions, this can be difficult.For example, when such an array is deployed onto the seabed, it isdifficult to predict how the packages will settle on to the seabed, andso the orientation of each package in three dimensional space is not ingeneral known.

To accurately extract the seismic signal, either the sensors must bepositioned so that they are in a constant position with respect to theearth's gravitational field, which would involve the use of mechanicalgimbals or the like to ensure that each package is orientated in apredetermined way, or the orientation of the sensors must be preciselyknown, which would typically involve the use of an attitude (or tilt)sensor. A variety of electromechanical attitude sensors exist, forexample accelerometers, mercury tilt meters, Micro-Electro-MechanicalSystems (MEMS) devices, etc.

The use of mechanical gimbals can significantly increase the complexityand size of each sensor package, and in certain deployments has beenfound to be unreliable. Accordingly, it is generally desirable to useattitude sensors to determine the attitude, or orientation, of eachpackage.

Up until recently, most existing seismic arrays use geophones andhydrophones based on electroacoustic technology, rather than fibre optictechnology, and the outputs from these sensors were multiplexed onto adatabus using local electronics for onward transmission. The outputsfrom the electromechanical attitude sensors were normally multiplexedonto the databus in the same way. The multiplexing system requiredelectrical power to be supplied through a cable to each package.

However, as mentioned above, in more recent times there has been muchinterest in the use of fibre optic sensors instead of the moretraditional electroacoustic sensors. One example is the so calledinterferometric fibre optic hydrophone or geophone, which operates byconverting an acoustic or seismic signal into a strain in a coil ofoptical fibre. This strain imposes a phase change in an optical signalpropagating through the coil, due to a combination of the physicallength change in the fibre and the stress-optic effect. In oneinterrogation scheme, this phase change is detected by beating thissignal with a reference signal of a slightly different frequency whichresults in the production of a beat frequency, or heterodyne carrier,equal to the difference in frequency of these two signals. The acousticsignal will therefore appear as a phase modulation on this carrier. Itwill be appreciated that other interrogation schemes may also be used,for example a Phase Generated Carrier scheme, which is homodyne based.

An example of a publication which discusses the prospect of using fibreoptic hydrophones and geophones for geophysical applications is anarticle entitled “Large Scale Multiplexed Fibre-Optic Arrays forGeophysical Applications” by Philip J Nash et al, Proceedings of SPIE(International Society for Optical Engineering), Industrial SensingSystems, 5-6 Nov. 2000, Boston, USA, Pages 55 to 65.

The advantage of an all optical approach is that no local multiplexingelectronics or electrical power is required within the packages toenable the fibre optic sensors to operate, as the optical outputs areall passively multiplexed within the array, for instance using timeand/or wavelength division multiplexing.

Accordingly, in an ideal situation, the attitude sensing of each packageshould also be carried out with an optical sensor which can be fittedinto the same multiplexing scheme. However, at the current time, no alloptical attitude sensors with the required performance are deemed tocurrently exist. Hence, whilst it is clear that an all optical seismicseabed array of fibre optic sensors will require attitude sensors, noneof the existing all-optical attitude sensors are likely to give therequired performance. Existing electromechanical attitude sensors willgive the required performance, but require their own multiplexing anddata transmission system, which will require a dedicated data link forthe attitude sensors, together with a supply of electrical power to eachof the packages to drive the electronic attitude sensors. This wouldclearly increase cabling requirements within the arrays, addsignificantly to their complexity, and detract from the generalperceived benefits described above of using an array consisting of fibreoptic sensors.

Accordingly, it would be desirable to provide an attitude sensing devicefor a package containing a fibre optic sensor which does not require itsown multiplexing and data transmission system, and which willaccordingly enable the benefits of an all-optical array of fibre opticsensors to be realised.

SUMMARY OF THE INVENTION

Viewed from a first aspect, the present invention provides an attitudesensing device for determining an attitude of a reference axis of apackage containing a fibre optic sensor, comprising: anelectro-mechanical attitude sensor for generating an electrical signalindicative of the attitude of that attitude sensor; and converter logicfor converting the electrical signal into a stimulus signal; the fibreoptic sensor being responsive to the stimulus signal to cause avariation in at least one predetermined property of an optical signaltransmitted through the fibre optic sensor, the attitude of thereference axis being determinable from the variation of thepredetermined property.

In accordance with the present invention, an attitude sensing device isprovided with an electromechanical attitude sensor which enables therequired accuracy and performance in attitude sensing to be achieved.However, rather than providing a dedicated data link for the attitudesensor to enable the attitude sensor's reading to be output, converterlogic is provided within the attitude sensing device for converting theelectrical signal output by the electromechanical attitude sensor into astimulus signal. This stimulus signal is of a type which the fibre opticsensor within the package is sensitive to, and accordingly the fibreoptic sensor is responsive to the stimulus signal to cause a variationin at least one predetermined property of an optical signal transmittedthrough the fibre optic sensor. The stimulus signal is encoded such thatthe attitude of the reference axis of the package is determinable fromthe variation of the predetermined property of the optical signal.

By this approach, the measurement made by the electromechanical attitudesensor is converted into a form where it is output using the standardmechanism provided for transmitting the outputs of the fibre opticsensor itself, and hence completely avoids the requirement to provide adedicated data transmission system to carry the output signal from theattitude sensor. Typically, the signals generated by the fibre opticsensor within the package are output over a fibre optic cable, and inaccordance with the present invention the output of theelectromechanical attitude sensor is also converted into a form where itcan be output by the fibre optic sensor over the fibre optic cable.

Hence, in summary, the present invention provides a high performance,low cost, reliable technique for integrating existing electromechanicalattitude sensors within a package containing a fibre optic sensor byinterfacing the attitude sensor with the existing all-optical seismicsensor and data transmission system. This then enables an array ofpackages to be produced to form a fully fibre optic array.

It will be appreciated that the optical signal transmitted through thefibre optic sensor can take a variety of forms, and may for example bein the visible, ultraviolet, or infrared range. In preferredembodiments, the optical signal is an infrared signal. Further, it willbe appreciated by those skilled in the art that the predeterminedproperty of the optical signal which is varied in dependence on thereceived stimulus signal may also take a variety of forms, dependent onthe construction of the fibre optic sensor, and for example may bephase, amplitude, polarisation, etc. In preferred embodiments, thepredetermined property is phase.

In preferred embodiments, to avoid the requirement to supply electricalpower to the package purely to power the electromechanical attitudesensor, the attitude sensing device is provided with a local powersource which is used to drive both the electro-mechanical attitudesensor and the converter logic.

It will be appreciated that the local power source could be of anysuitable form, so long as it provides for the generation of thenecessary electrical power within the attitude sensing device. Hence,the local power source could be provided by a battery, or alternativelycould be a device which generates electrical power based on a receivedstimulus, for example optical power generated remotely and sent to theattitude sensing device for conversion into electrical power.

As discussed earlier, the fibre optic sensor may take a variety offorms, and the type of stimulus signal generated by the converter logicwill depend on the type of fibre optic sensor included within thepackage. However, in preferred embodiments, the fibre optic sensor is avibration sensor, and the converter logic comprises: control logic forgenerating a drive signal dependent on the electrical signal generatedby the electro-mechanical attitude sensor; and a vibration source forreceiving the drive signal and generating as the stimulus signal asequence of vibrations dependent on the drive signal; whereby the fibreoptic vibration sensor is responsive to the sequence of vibrations tocause the variation in the at least one property of the optical signal.The term “vibration sensor” as used herein is intended to broadly coverany fibre optic sensor which is sensitive to vibrations, and hence, forexample, the vibration sensor may be a geophone used to detectdirectional vibration (for example an accelerometer or a displacementsensor), or a hydrophone which is sensitive to omnidirectionalvibrations in order to generate pressure information.

It will be appreciated by those skilled in the art that the drive signalgenerated by the control logic may take a variety of forms dependent onhow the sequence of vibrations is to encode the attitude measurement.For example, the sequence of vibrations from the vibration source may beencoded digitally, such that a predetermined sequence of ones and zerosare used to encode the attitude measurement. In such cases, the drivesignal will clearly cause the vibration source to be turned on and offin a predetermined sequence. However, alternatively, the sequence ofvibrations from the vibration source may be encoded using vibrations ofvarying amplitude, and in such cases the drive signal will typically beof a variable voltage used to control the amplitude of vibration of thevibration source at any particular point in time. A further alternativeis for the sequence of vibrations from the vibration source to beencoded using vibrations of varying frequency, and in such cases it isclear that the drive signal generated by the control logic will specifythe frequency of vibration to be generated by the vibration source atany particular point in time. Yet another alternative is for thesequence of vibrations from the vibration source to be encoded usingvibrations of varying duration, in which case the drive signal will bearranged to turn the vibration source on and off for predeterminedperiods of time dependent on the attitude measurement. It will beappreciated by those skilled in the art that there will indeed befurther alternative ways of encoding the sequence of vibrationsgenerated by the vibration source, and the above are intended merely toprovide examples of suitable implementations.

In preferred embodiments of the invention, the control logic is arrangedto generate a drive signal which is directly indicative of the attitudeof the attitude sensor as indicated by the electrical signal output fromthe attitude sensor, such that the variation in the predeterminedproperty of the optical signal is directly indicative of the attitudemeasured by the attitude sensor. Since the orientation of the attitudesensor within the package will be fixed at manufacture, there willclearly be a direct relation between the attitude of the attitude sensorand the attitude of the reference axis of the package, and accordinglyit is clear that the attitude of the reference axis can readily bedetermined from the measured attitude of the attitude sensor. Indeed, inpreferred embodiments the attitude sensor will actually be aligned alongthe reference axis, such that the attitude of the attitude sensorequates directly to the attitude of the reference axis.

However, in situations where the attitude of the attitude sensor doesnot directly equate to the attitude of the reference axis, the controllogic of some embodiments may be arranged to perform a calculation ofthe attitude of the reference axis based on the electrical signalreceived from the attitude sensor indicative of the attitude of theattitude sensor, and to then generate a drive signal which is directlyindicative of the attitude of the reference axis. However, in preferredembodiments, it is desirable to keep the complexity of the control logicto a minimum, and hence it is envisaged that the control logic ofpreferred embodiments will generate a drive signal which correspondsdirectly to the electrical signal generated by the attitude sensor, andhence causes a sequence of vibrations, and ultimately the variation ofthe predetermined property of the optical signal, to be dependent on theattitude of the attitude sensor. If any alteration of that attitudevalue is required in order to arrive at the attitude of the referenceaxis, it is envisaged that in preferred embodiments this would be doneby external processing logic, for example processing logic to which anarray of such packages is coupled.

It will be appreciated by those skilled in the art that the vibrationsource could take a variety of forms. However, in preferred embodimentsthe vibration source is a piezoelectric vibrator or electric motor.Alternatively, the vibration source could be an electrostrictivevibrator, a magnetostrictive vibrator, etc.

As mentioned earlier, the fibre optic sensor may take a variety offorms, and in preferred embodiments there will typically be more thanone fibre optic sensor within the package, and indeed more than one typeof fibre optic sensor within the package. However, in preferredembodiments, the fibre optic sensor within the package which responds tothe stimulus signal generated by the converter logic is a geophone. Inpractice, this converter logic will be positioned in the vicinity of thefibre optic sensor responding to the stimulus signal, and it isgenerally more desirable for that fibre optic sensor to be a geophone,since a hydrophone needs to be located in fluid contact with the fluidwhose pressure is being measured, and hence is often located in aharsher environment than the geophones.

Preferably, the power source takes the form of a battery, which may ormay not be rechargeable, depending on, for example, the planneddeployment environment. However, in preferred embodiments, the batteryis rechargeable, and the attitude sensing device further comprises anopto-electronic converter coupled to the battery, and arranged toreceive an optical charge signal transmittable to the opto-electronicconverter via an optical fibre, the opto-electronic converter beingresponsive to the optical charge signal to generate a current used tocharge the battery. As mentioned earlier, a typical deploymentapplication will involve the use of an array of packages connected byfibre optic cables. By the above approach, one of the optical fibres canbe used for the transmission of power to the attitude sensing device toenable the battery to be trickle charged. In preferred embodiments, theopto-electronic converter is a photodiode coupled across the terminalsof the battery.

Depending upon the deployment of the package, only a single measurementof attitude may be required, for example immediately after deployment,or alternatively attitude measurements may be required at predeterminedintervals. Typically, in deployments such as the seismic seabed arraydeployment mentioned earlier, the intervals between measurements may bequite large, and for example measurements of attitude might be taken atapproximately yearly intervals, to ensure that no significant changes inattitude of any of the sensors is occurring.

Hence, in one embodiment, the attitude sensing device may furthercomprise a timer for determining when to switch on the attitude sensorto cause the electrical signal indicative of the attitude to begenerated. This simple implementation may be sufficient for manyapplications. However, if more control is required as to when to takeattitude measurements, the attitude sensing device may be provided witha receiver for receiving a command signal, the receiver being responsiveto the command signal to switch on the attitude sensor to cause theelectrical signal indicative of the attitude to be generated. By thisapproach, the attitude sensor can be turned on as and when required.

However, given the earlier discussions, it would clearly be desirable toavoid the need to provide any dedicated cabling to the attitude sensingdevice merely to enable the command signal to be passed to the receiver.Accordingly, in one embodiment, the command signal may be formed as apredetermined vibration sequence, and the receiver may be formed by thevibration source, the vibration source being further arranged to convertthe received command signal into an electrical signal used to switch onthe attitude sensor. Hence, in such embodiments, the vibration sourcehas a bi-directional operation, such that it can convert an electricalsignal (i.e. the drive signal) into a vibration sequence, and converselyconvert vibrations into a suitable electrical signal to be routed to theattitude sensor to turn it on and off.

Alternatively, the receiver could be formed by a separate device to thevibration source, for example a separate electroceramic hydrophone, or ageophone.

In an alternative embodiment, the receiver may be formed by an opticalreceiver, and the command signal may be formed as an optical commandsignal transmittable to the optical receiver via an optical fibre. Insuch cases, the optical receiver will be arranged to convert the opticalcommand signal into an electrical signal to be sent to the attitudesensor to turn it on and off. In preferred embodiments where the batteryis also trickle charged via an optical charge signal passed through anoptical fibre, that same optical fibre may also be used for thetransmission of the optical command signal to the optical receiver.

Whilst one electromechanical attitude sensor may be sufficient incertain deployments, a more complete determination of the attitude ofthe reference axis may be made by including an additionalelectro-mechanical attitude sensor. Accordingly, in preferredembodiments, the attitude sensing device further comprises a furtherelectro-mechanical attitude sensor for generating an electrical signalindicative of the attitude of that further attitude sensor, the furtherattitude sensor being mounted at an angle with respect to the attitudesensor, the converter logic being arranged to receive the electricalsignals from both attitude sensors, and to generate a single drivesignal dependent on those electrical signals which is used to generatethe sequence of vibrations. In one preferred embodiment, theelectromechanical attitude sensors are orthogonally mounted with respectto each other.

In preferred embodiments, a coding scheme is used for the sequence ofvibrations from the vibration source such that the attitude of eachattitude sensor is represented independently within the sequence ofvibrations. As an example, the coding scheme may be a time divisionmultiplex scheme. However, it will be appreciated by those skilled inthe art that alternatively coding schemes may be used, for examplefrequency division, etc.

In an alternative embodiment, the converter logic may be arranged toperform some processing on the electrical signals from both attitudesensors, in order to determine the attitude of the reference axis, andin that event the single drive signal generated will be used to generatea stimulus signal which is directly indicative of the attitude of thereference axis.

Viewed from a second aspect, the present invention provides a packagecomprising one or more fibre optic sensors and an attitude sensingdevice in accordance with the first aspect of the present invention.

Viewed from a third aspect, the present invention provides an array ofpackages coupled by a fibre optic cable, each package comprising one ormore fibre optic sensors coupled to the fibre optic cable, and anattitude sensing device in accordance with the first aspect of thepresent invention.

Viewed from a fourth aspect, the present invention provides a method ofdetermining an attitude of a reference axis of a package containing afibre optic sensor, comprising the steps of: (i) employing anelectromechanical attitude sensor within the package to generate anelectrical signal indicative of the attitude of that attitude sensor;(ii) converting, within the package, the electrical signal into astimulus signal; (iii) arranging the fibre optic sensor to be responsiveto the stimulus signal to cause a variation in at least onepredetermined property of an optical signal transmitted through thefibre optic sensor; and (iv) determining the attitude of the referenceaxis from the variation of the predetermined property.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described further, by way of example only,with reference to preferred embodiments thereof as illustrated in theaccompanying drawings, in which:

FIG. 1 is a diagram illustrating a deployment of a seismic seabed arrayof an embodiment of the present invention;

FIG. 2 is a diagram illustrating the configuration of a fibre optichydrophone;

FIGS. 3A and 3B are diagrams illustrating the configuration of a fibreoptic geophone;

FIG. 4 is a diagram schematically illustrating a package incorporatingfibre optics sensors and an attitude sensing device in accordance withpreferred embodiments of the present invention;

FIG. 5 is a block diagram illustrating the components of the controlelectronics illustrated in FIG. 4; and

FIG. 6 is a block diagram illustrating more details of the attitudesensing device of preferred embodiments of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is a diagram illustrating a deployment of a fibre optic seabedseismic array in accordance with an embodiment of the present invention.The array consists of a plurality of packages 50 coupled by a fibreoptic cable 55. The array is deployed on the seabed 40, and depending onthe depth of the seabed 40 below the sea surface 30, this deployment maybe performed by divers positioning each package 50 on the seabed, or bythe use of submersible vehicles to perform such positioning, or thearray may be directly deployed from the surface without assistance atthe seabed.

Attached to one end of the fibre optic cable 55 will be an opticalsignal source such as a laser for propagating an optical signal alongthe fibre optic cable 55, and some receive circuitry for detecting thesignals returned from the sensors within each of the packages 50. Thisoptical signal source and receive circuitry is not illustrated in FIG.1, but would typically be located at some convenient location, forexample a boat, oilrig, etc located on the sea surface 30.

When it is desired to carry out a reservoir characterisationmeasurement, one or more acoustic sources 20 are used to transmitacoustic signals 60 into the seabed structure 40, and the array ofpackages 50 are used to record the signals reflected from the variousgeological layers within the seabed structure 40. Typically a pluralityof acoustic sources 20 are used during such measurements, and may forexample be trailed behind a boat 10 on the sea surface 30.

The operation of the optical fibre sensors within each package 50 willbe well understood by those skilled in the art, and hence will not bediscussed in detail herein. However, a brief discussion of the operationof the hydrophones and geophones which may be deployed within eachpackage 50 will now be provided with reference to FIGS. 2 and 3.

FIG. 2 shows an example of an interferometric fibre optic hydrophone 200which may be used to detect pressure, and in particular in thedeployment of FIG. 1, the pressure of the seawater adjacent thehydrophone. The hydrophone 200 basically consists of an optical fibre210 wound around a mandrel 215 such as an air-backed mandrel. An arrayof these coils may be spliced together, separated by a directionalcoupler 240, 250 with a reflective mirror 220, 230 attached to one port.The other port of the coupler is index matched such that reflection ofan optical signal passing along the optical fibre 210 only occurs in onedirection, ensuring that multi-path reflections are suppressed.

The fibre optic hydrophone 200 operates by converting an acoustic orseismic signal 60 into a strain in the coil of optical fibre 210. Thisimposes a phase change in an optical signal propagating through the coil210, due to a combination of the physical length change in the fibre andthe stress-optic effect. This phase change is detected by beating thesignal with a reference signal of a slightly different frequency whichresults in the production of a beat frequency, or heterodyne carrier,equal to the difference in frequency of these two signals. The acousticsignal will therefore appear as a phase modulation on this carrier. Itwill be appreciated by those skilled in the art that other interrogationtechniques, such as Phase Generated Carrier, could also be applied.

As will also be appreciated by those skilled in the art, variousarchitectures have been proposed for enabling a plurality of suchhydrophones to be spliced serially together, and for the individualsignals from each hydrophone to be detected. For example time divisionmultiplexed (TDM) architectures have been proposed, and in additioncombined time and wavelength division multiplexing architectures havebeen proposed. Both of these approaches are discussed in some detail inthe article entitled “Large Scale Multiplexed Fibre-Optic Arrays forGeophyiscal Applications” by Philip J Nash et al, Proceedings of SPIE(International Society for Optical Engineering), Industrial SensingSystems, 5-6 Nov. 2000, Boston, USA, Pages 55 to 65. Suitablemultiplexing techniques are also discussed in PCT patent application no.PCT/GB00/01300 (publication no. WO 00/62021).

It will be appreciated that FIG. 2 illustrates just one example of afibre optic hydrophone that may be used within the packages 50, and itwill be appreciated by those skilled in the art that other arrangementsof fibre optic hydrophone can be used.

FIGS. 3A and 3B illustrate an example of a fibre optic geophone whichcan be used to detect directional vibration. The geophone basicallyconsists of a seismic mass 310 located on a flexible plate 320 within anenclosure 300. The flexible plate 320 is rigidly connected to theenclosure 300 by a support member 340, which in the example of FIG. 3Ais shown as being connected to the middle of the flexible plate. Anoptical fibre coil 330 is then wound onto at least one side of theflexible plate 320 in the manner illustrated in FIG. 3B, which is a viewshowing the underside of the flexible plate 320.

As shown in FIG. 3B, a proportion of an optical signal passing throughoptical fibre 325 is passed into the optical fibre coil 330 of thegeophone via a coupler 305. At the inner end of the coil 330, areflective end 315 is provided, which causes the optical signal to bereflected back through the coil and back onto the main optical fibre 325via the coupler 305.

As will be appreciated by those skilled in the art, vibrations along theaxis of the support member 340 will cause the flexible plate 320 to flexdue to the presence of the seismic mass 310, this flexing causing astrain in the fibre optic coil 330, which in a similar manner to theearlier described hydrophone will impose a phase change in an opticalsignal propagating through the coil 330. As will be appreciated by thoseskilled in the art, similar techniques as those discussed earlier withrespect to the hydrophone of FIG. 2 are used to detect this phase changeand hence determine the vibrations experienced by the geophone 300.

As will be appreciated by those skilled in the art, geophones such asthose illustrated in FIGS. 3A and 3B can be spliced together usingappropriate directional couplers, and indeed in a typical package 50there will typically be three orthogonally mounted geophones 300 allconnected to the same optical fibre to enable directional vibrationmeasurements to be taken in three orthogonal directions.

As discussed earlier, when deploying packages 50 containing one or morefibre optic sensors, it is important to know the orientation of eachindividual package in order to be able to correctly analyse the signalsoutput by the fibre optic sensors within the package. In manydeployments, such as the deployment illustrated in FIG. 1, it isdifficult to predict the orientation of the packages 50, and accordinglyan attitude sensor may be required for each package in order to generatea signal indicative of the attitude of each package 50, and hence theattitude of the various sensors within the package.

FIG. 4 is a diagram illustrating a package 50 incorporating an attitudesensing device in accordance with preferred embodiments of the presentinvention. As shown in FIG. 4, a package 50 may in preferred embodimentscontain three orthogonally mounted geophones 300, and one hydrophone200, with the package 50 being coupled to other packages via a fibreoptic cable 55. Although the hydrophone 200 is shown entirely within thepackage 50, it will be appreciated that it will physically need to belocated in fluid contact with the sea water to enable the pressure to bemeasured.

The orientation of the geophones and hydrophone within the package 50will be fixed, but in order to determine the orientation or attitude ofeach such fibre optic sensor, it is first necessary to know the attitudeof a reference axis 65 of the package 50 within three dimensional space.In preferred embodiments, this determination is enabled by the presenceof the attitude sensing device 400 within the package 50.

The attitude sensing device 400 of preferred embodiments preferablyconsists of one or more electromechanical attitude sensors 430, whichmay be provided by any one of a number of known electromechanicalattitude sensors, for example accelerometers, mercury tilt meters, MEMSdevices, etc. The attitude sensor 430 will generate an electrical signalindicative of the attitude of that attitude sensor, which is input tocontrol electronics 440, which are used in preferred embodiments togenerate a drive signal for a piezoelectric vibrator 450 dependent onthe electrical signal received from the attitude sensor 430. Thepiezoelectric vibrator 450 is responsive to the drive signal to generatea sequence of vibrations 460 which are then detected by one of the fibreoptic sensors 200, 300 within the package 50. Those vibrations will thenbe converted by the fibre optic sensor 200, 300 into a strain in thecoil of optical fibre within that fibre optic sensor, resulting in aphase change in an optical signal propagating through the coil, which isindicative of the attitude measurement. By this mechanism, it will beappreciated that it is possible to output the attitude measurement fromthe electromechanical attitude sensor 430 using the standard fibre opticcable 55 connected to the package 50. This removes the need to provide aseparate dedicated data transmission system to enable the output of theelectromechanical attitude sensor to be output to the receiver andprocessing stages located at the far end of the optical fibre cable 55.

In preferred embodiments, the attitude sensor 430 and controlelectronics 440 are powered by a local battery 410 provided within theattitude sensing device 400. Further, in preferred embodiments, a switch420 is provided for switching on the attitude sensor 430 and controlelectronics 440 at predetermined intervals, or in response to a commandsignal. More details of the battery 410 and switch 420 of preferredembodiments will be described later with reference to FIG. 6.

In preferred embodiments, the piezoelectric vibrator 450 is preferablypositioned next to one of the fibre optic sensors 200, 300, and inpreferred embodiments is located next to one of the geophones 300. Inpractice, location of the piezoelectric vibrator 450 next to thehydrophone 200 is less desirable, since the hydrophone typically needsto be connected in fluid contact with the fluid whose pressure is beingmeasured, for example the sea in the FIG. 1 implementation, and this isa more hostile environment than that to which the geophones 300 areexposed, since the geophones 300 can be located entirely within thepackage 50 and sealed from the sea.

In preferred embodiments, two electromechanical attitude sensors,positioned orthogonally with respect to each other, are provided withinthe attitude sensing device 400, as this enables a more completemeasurement of attitude to be determined for some types of attitudesensor. For example, certain types of attitude sensor may lose accuracy(or stop working altogether) after a certain tilt angle is exceeded, andhence the use of two sensors increases the range of angles over whichthe attitude can be measured. Further, in preferred embodiments, thesetwo attitudes sensors are located adjacent to one of the geophones, withthe piezoelectric vibrator 450 of the attitude sensing device 400 beingarranged to generate vibrations in the direction to which that geophoneis sensitive.

More details of the control electronics 440 used in preferredembodiments where two attitude sensors 430 are provided will now bediscussed with reference to FIG. 5.

As shown in FIG. 5, the output from each attitude sensor 430 is passedto a corresponding buffer amplifier 500, 540, which can buffer thatoutput and then amplify it prior to passing the amplified version of theattitude sensor's output signal to a corresponding analogue to digital(A/D) converter 510, 550. The outputs from the analogue to digitalconverters 510, 550 are then passed to an encoder/multiplexer unit 520,which is preferably provided by a digital signal processor (DSP). In aparticular preferred embodiment, the digital signal processor takes theform of a field programmable gate array (FPGA) which is arranged togenerate a suitable drive signal for the piezoelectric vibrator 450dependent on the two input signals from the attitude sensors.

It will be appreciated that the encoder/multiplexer unit 520 may bearranged to perform any suitable encoding. For example, since therelative orientation of each attitude sensing device with respect to thereference axis of the package will be predetermined, it is possible thatthe encoding/multiplexing unit could be arranged to determine from thereceived signals the actual attitude of the reference axis, and todirectly encode that reference axis attitude in the output drive signal.However, in preferred embodiments, it is desirable to keep thecomplexity of the encoding/multiplexing unit 520 to a minimum, and soinstead the encoding/multiplexing unit 520 generates two encoded signalscorresponding to the two received signals from the attitude sensors, andthen uses a coding scheme such as time division multiplexing, frequencydivision multiplexing, etc, in order to generate a single drive signalto be used to drive the piezoelectric vibrator. In preferredembodiments, the single drive signal is output to a buffer amplifier530, which can buffer that output drive signal, and then amplify itprior to providing it to the piezoelectric vibrator 450.

It will also be appreciated that there are a number of ways in which theencoder/multiplexer 520 could encode the signals received from theattitude sensors. For example, the outputs from each attitude sensorcould be encoded either digitally or in analogue form. In preferredembodiments, the signals are encoded in digital form in order to producetwo ten bit digital signals, each ten bit signal encoding the attitudeof the corresponding attitude sensor. These two ten bit digital signalsare then coded using time division or frequency division multiplexingtechniques to generate a single drive signal for the piezoelectricvibrator, which will cause the piezoelectric vibrator 450 to turn on andoff dependent on the digital output signal from the encoder/multiplexerunit 520.

As an alternative to encoding the output signals from each attitudesensor in a digital form, the encoder/multiplexer 520 may be arranged toencode each signal in an analogue form, and hence as an example maygenerate drive signals which cause the sequence of vibrationssubsequently produced by the piezoelectric vibrator 450 to have varyingamplitude, varying frequency, varying duration, etc, dependent on theattitude measured by each attitude sensor.

In the above example, it is assumed that the output from each of theattitude sensors is generated in an analogue form. In general, this maybe a simple analogue voltage with a level dependent on the tilt of theattitude sensor. However, in some embodiments, the analogue output mayhave a more complex form, or alternatively the attitude sensors may bearranged to directly generate a digital output. In the event that theattitude sensors generate digital outputs, it will be clear that the A/Dconverters 510, 550 will no longer be required. It will be appreciatedthat the encoder/multiplexer unit 520 will need to be designed to handlethe particular data format generated by the attitude sensors. Further,it will be appreciated that, irrespective of whether the outputsgenerated by the attitude sensors are in analogue or digital form, thedrive signal(s) generated by the control electronics 440 can be eitherin an analogue or a digital form depending on the vibrator 450 to bedriven.

FIG. 6 is a diagram illustrating in more detail the construction of thebattery unit 410 and the switch unit 420 of preferred embodiments. Inpreferred embodiments, the battery unit 410 incorporates a rechargeablebattery 640 which has a photodiode 620 coupled across its terminals inseries with a voltage regulator 630. An optical fibre 610 within theoptical fibre cable 55 can then be used as a “charging fibre” togenerate an optical charge signal which is routed via a directionalcoupler 670 to the photodiode 620 in order to cause the generation of acurrent to trickle charge the battery 640. In preferred embodiments,this optical charge signal used to recharge the battery is generated bya pump laser 600. Typically, this pump laser 600 will be a differentlaser to the one used to generate the optical signal passed through thegeophones and hydrophones.

As regards the switch 420, this may take a variety of forms. Forexample, it may merely involve a switch being coupled to a timer, suchthat when the timer expires, the switch couples the attitude sensor andcontrol electronics to the battery in order to cause an attitudemeasurement to be taken. However, in preferred embodiments the switchunit 420 includes a switch 660 which is connected to a command receiver650. The command receiver 650 is an optical receiver (in preferredembodiments incorporating a photodiode) which is arranged to receive anoptical command signal transmitted over the optical fibre 610, androuted to the command receiver 650 via a coupler 680. The opticalcommand signal would typically comprise a particular sequence of opticalpulses used to trigger the command receiver 650 to output an electricalsignal to the switch 660 to cause the switch to couple the battery 640to the attitude sensor(s) 430 and the control electronics 440. Inpreferred embodiments, the same optical fibre 610 can be used fortransmission of the optical charge signal to the photodiode 620 and theoptical command signal to the command receiver 650. Furthermore, by useof appropriate couplers 670, 680, the optical fibre 610 may pass throughmultiple packages, and in particular through the attitude sensingdevices of multiple packages in order to provide appropriate signals tothe battery 410 and switch 420 units of those attitude sensing devices,the various signals being multiplexed in an appropriate manner, e.g.time division, frequency division, etc.

In an alternative embodiment, instead of transmitting the opticalcommand signal along the same optical fibre as used to carry the opticalcharge signal, the optical command signal could be transmitted along thetelemetry optical fibre used by the hydrophones and geophones at awavelength different to the hydrophone/geophone interrogation unit, andthen “tapped off” from this fibre using a wavelength divisionmultiplexing (WDM) coupler. As an example, if the hydrophones andgeophones are interrogated at 1550 nm, it may be possible to send theoptical command signal at 1480 nm, and use a 1480/550 nm WDM coupler totap the optical command signal off to the command receiver 650illustrated in FIG. 6.

As an alternative to the command receiver 650, the piezoelectricvibrator 450 of FIG. 4 may actually be a bidirectional device, which caneither convert the electrical drive signal from the control electronics440 into a vibration sequence 460, or instead can receive a commandsignal in the form of a vibration command signal, and convert that intoan electrical signal to send to the switch 660 to turn on the attitudesensor(s) 430 and the control electronics 440. Alternatively the commandsignal may be received by a separate, dedicated acoustic or seismicreceiver.

Accordingly, it can be seen that the attitude sensing device ofpreferred embodiments of the present invention alleviates the earlierdescribed problems of attitude sensing in all-optical arrays, with aminimum of additional electronics and data transfer requirements. Thisavoids the requirement to use a complex optical attitude sensor whichmay give lower performance, or the provision of a separate datatransmission system which would typically otherwise be required ifelectromechanical attitude sensors were used. The approach of thepreferred embodiment uses the existing fibre optic sensors, telemetryand multiplexing system to recover the information from conventionalelectromechanical attitude sensors. The approach of the preferredembodiment of the present invention enables an attitude sensing deviceto be provided in fibre optic sensor packages which is of lower cost andhigher reliability than other known attitude sensing techniques.

Although a particular embodiment of the invention has been describedherein, it will be apparent that the invention is not limited thereto,and that many modifications and additions may be made within the scopeof the invention. For example, various combinations of the features ofthe following dependent claims could be made with the features of theindependent claims without departing from the scope of the presentinvention.

1. An attitude sensing device for determining an attitude of a referenceaxis of a package containing a fibre optic sensor, comprising: anelectromechanical attitude sensor for generating an electrical signalindicative of the attitude of that attitude sensor; and converter logicfor converting the electrical signal into a stimulus signal; the fibreoptic sensor being responsive to the stimulus signal to cause avariation in at least one predetermined property of an optical signaltransmitted through the fibre optic sensor, the attitude of thereference axis being determinable from the variation of thepredetermined property.
 2. An attitude sensing device as claimed inclaim 1, further comprising: a power source for the electromechanicalattitude sensor and the converter logic.
 3. An attitude sensing deviceas claimed in claim 1, wherein the fibre optic sensor is a vibrationsensor, and the converter logic comprises: control logic for generatinga drive signal dependent on the electrical signal generated by theelectromechanical attitude sensor; and a vibration source for receivingthe drive signal and generating as the stimulus signal a sequence ofvibrations dependent on the drive signal; whereby the fibre opticvibration sensor is responsive to the sequence of vibrations to causethe variation in the at least one property of the optical signal.
 4. Anattitude sensor as claimed in claim 3, wherein the sequence ofvibrations from the vibration source is encoded digitally.
 5. Anattitude sensor as claimed in claim 3, wherein the sequence ofvibrations from the vibration source is encoded using vibrations ofvarying amplitude.
 6. An attitude sensor as claimed in claim 3, whereinthe sequence of vibrations from the vibration source is encoded usingvibrations of varying frequency.
 7. An attitude sensor as claimed inclaim 3, wherein the sequence of vibrations from the vibration source isencoded using vibrations of varying duration.
 8. An attitude sensingdevice as claimed in any of claims 3, wherein the vibration source is apiezoelectric vibrator.
 9. An attitude sensing device as claimed inclaim 1, wherein the fibre optic sensor is a geophone.
 10. An attitudesensing device as claimed in claim 2, wherein the power source is abattery.
 11. An attitude sensing device as claimed in claim 10, furthercomprising an opto-electronic converter coupled to the battery, andarranged to receive an optical charge signal transmittable to theopto-electronic converter via an optical fibre, the opto-electronicconverter being responsive to the optical charge signal to generate acurrent used to charge the battery.
 12. An attitude sensing device asclaimed in claim 1, further comprising a timer for determining when toswitch on the attitude sensor to cause the electrical signal to begenerated.
 13. An attitude sensing device as claimed in claim 1, furthercomprising a receiver for receiving a command signal, the receiver beingresponsive to the command signal to switch on the attitude sensor tocause the electrical signal to be generated.
 14. (canceled)
 15. Anattitude sensor as claimed in claim 13, wherein the receiver is anoptical receiver and the command signal is an optical command signaltransmittable to the optical receiver via an optical fibre.
 16. Anattitude sensing device as claimed in claim 1, further comprising: afurther electromechanical attitude sensor for generating an electricalsignal indicative of the attitude of that further attitude sensor, thefurther attitude sensor being mounted at an angle with respect to theattitude sensor, the converter logic being arranged to receive theelectrical signals from both attitude sensors, and to generate a singledrive signal dependent on those electrical signals which is used togenerate the sequence of vibrations.
 17. (canceled)
 18. An attitudesensing device as claimed in claim 17, wherein the coding scheme is atime division multiplexed scheme.
 19. A package comprising: one or morefibre optic sensors; and an attitude sensing device as claimed inclaim
 1. 20. An array of packages coupled by a fibre optic cable, eachpackage comprising one or more fibre optic sensors coupled to the fibreoptic cable, and an attitude sensing device as claimed in claim
 1. 21. Amethod of determining an attitude of a reference axis of a packagecontaining a fibre optic sensor, comprising the steps of: (i) employingan electromechanical attitude sensor within the package to generate anelectrical signal indicative of the attitude of that attitude sensor;(ii) converting, within the package, the electrical signal into astimulus signal; (iii) arranging the fibre optic sensor to be responsiveto the stimulus signal to cause a variation in at least onepredetermined property of an optical signal transmitted through thefibre optic sensor; and (iv) determining the attitude of the referenceaxis from the variation of the predetermined property. 22-24. (canceled)25. An attitude sensing device as claimed in claim 3, further comprisinga receiver for receiving a command signal, the receiver being responsiveto the command signal to switch on the attitude sensor to cause theelectrical signal to be generated.
 26. An attitude sensing device asclaimed in claim 25, wherein the command signal comprises apredetermined vibration sequence and the receiver is formed by thevibration source, the vibration source being further arranged to convertthe received command signal into an electrical signal used to switch onthe attitude sensor.
 27. An attitude sensing device as claimed in claim3, further comprising: a further electromechanical attitude sensor forgenerating an electrical signal indicative of the attitude of thatfurther attitude sensor, the further attitude sensor being mounted at anangle with respect to the attitude sensor, the converter logic beingarranged to receive the electrical signals from both attitude sensors,and to generate a single drive signal dependent on those electricalsignals which is used to generate the sequence of vibrations.
 28. Anattitude sensing device as claimed in claim 27, wherein a coding schemeis used for the sequence of vibrations from the vibration source suchthat the attitude of each attitude sensor is represented independentlywithin the sequence of vibrations.